var t;
//點擊滾動事件
function gun(){
    var a = window.pageYOffset;
    if(a-100<0){
        window.clearInterval(t);
    }
    window.scrollTo(0,a-100);
}
//拖動顯示圖標
$(window).scroll(function(){
    var a = window.pageYOffset;

    if(a>200){
        $('#returnTop').css('display','block');
    }
    else{
        $('#returnTop').css('display','none');
    }
})